Rolling wheel distance measuring devices including devices which employ some form of digital counter are known in the prior art. Ordinarily, in the prior art, the counter mechanism is mechanically operated in response to the rotation of a measuring wheel having a known circumference. These mechanical devices vary considerably in their complexity and some are quite complex and costly.
The present invention seeks to improve on the known prior art by providing a rolling wheel measuring device having an electrically operated counter which responds to the activation of a relatively stationary magnetic switch on the device which is positioned to be cyclically operated by the movement of permanent magnets arranged in predetermined spaced relationship on a rotating carrier attached to the wheel. The cost and relative complexity of mechanical means for activating a counter are dispensed with.
A significant feature of the invention resides in the placement between the magnetic switch and rotating magnets of a shield formed of magnetically attractable material. In a forward mode of operation of the device, the shield assumes a non-active position away from the switch, but in a reverse mode of operation, the shield is magnetically turned to a switch-shielding position in relation to the rotating magnets so that the switch cannot activate the digital counter.